Heavy metals

Heavy metals are occurring in the environment naturally and most of them are important to physiological and biochemical reactions in animal bodies and plants. For example, metals as manganese, zinc, nickel and copper plays as cofactors important roles for many enzymes and proteins. However, there are also heavy metals being not so important in biological re-actions. Metals as aluminum, chromium, arsenic, cadmium, mercury and lead have instead toxic effects toward species. (Chen et al. 2016.) The industrial and agricultural activity dur-ing the centuries have affected the unnatural releasdur-ing of heavy metals from the soil and rock.

Because of the wide distribution, heavy metals are under public concern. (Tchounwou et al.

2012). Heavy metals being useful in biological reactions are also harmful in high amounts.

This is because useful heavy metals are required to biological reactions only in trace levels.

(Chen et al. 2016.)

Because of the non-degradable properties, heavy metals tend to accumulate to soils and sed-iments. Bioaccumulation to marine organisms and plants happens too and, in this way, heavy metals become richer in the food chain causing harmful effects also to humans. Because of the accumulation and toxic properties, heavy metals are considered to environmentally harmful and hazardous substances being threat to public health (Coelho et al. 2018). There are various toxic effects of heavy metals towards biological systems. Part of the effects are still unknown, and more research is still done to find out them. Most of the toxic effects towards biological systems are related to the damaging of cellular organelles. Heavy metals expose to DNA damages and changes in the conformational functions, which may cause carcinogenic actions. (Tchounwou et al. 2012).

Biological, physical and chemical factors affect to the toxicity and availability of heavy met-als. This means for example how heavy metals act between phases and transport inside to organisms having in the same time toxic effects. (Coelho et al. 2018). For example, the bio-availability of heavy metals is controlled by physical factors such as temperature.

(Tchounwou et al. 2012). In addition, also organic carbon content of the soil and pH affect to bioavailability of heavy metals. (Römkens et al. 20019). On the other hand, biological

factors as specie characteristics and biochemical adaptation define how heavy metals affect to different species. Complexation kinetics, thermodynamic equilibrium and lipid solubility are for example the chemical factors. (Tchounwou et al. 2012).

To consider heavy metals in forest industry, there are several different sources of heavy metals to enter to forest industry processes. The most typical way to enter processes are via raw material wood. Wood may contain natural heavy metals accumulated from soil. The other way to enter processes are via raw water. In addition, process waters may also contain metals from the pipelines dissolved by the corrosion. If the process chemicals contain heavy metals, chemicals may also be one source to metals too. (Knowpap, 2016a).

In Table 5 below, contents of heavy metals in three different cases are listed. In the Ala-kangas (2000) report, heavy metal contents in wood used as fuel are listed. However, these contents include also contents in roots, needles, branch and bark, so the values are not com-parable to contents which may appear in forest industry where only trunk tree is used. In the study of Harju et al. (1997), heavy metal contents in spruce were measured near metal fac-tory in Harjavalta and also in Merimasku, which is located far away from the metal facfac-tory.

As can be seen, also the location of wood for example near industry may have an effect to heavy metal contents in wood. Contents were measured from tree trunk.

Table 5. Different heavy metal contents in three different cases. In the first column, contents in wood fuel are listed, two following columns list the effect of the location of wood to heavy metal content in trunk. All the contents are reported as mg/kg.

Heavy metal

A very good example of heavy metals presence in forest industry processes is a VTT’s (1998) study of heavy metal concentrations in sulphite pulp mill. The aim of the study was to find out the circulation flows of heavy metals in the processes and to point the most significant

sources of heavy metals to the process. The main heavy metals studied were aluminum, cal-cium, cadmium, chromium, lead and mercury. The results showed the source for metals as lead and cadmium to be wood chips in which the cadmium content was 90 - 141 µg/kg and the lead content 229 - 339 µg/kg. Metals as Cd and Pb also became richer in soda recovery boiler. (VTT, 1998).

Keitaanniemi (1979) has studied entering of metals as calsium Ca, magnesium Mg, silica Si, aluminium Al, manganese Mn, natrium Na and ferrous Fe to the processes of kraft pulp mill.

The study was made by having measurements in four different kraft pulp mills in Finland.

The study showed the main sources for metals to be wood as a raw material, but also raw water and some process chemicals. Average values for metals entering to the processes of kraft pulp mills were following:

Table 6. Average value (kg per ton of non-bleached, unscreened pulp) of metals entering to kraft pulp mills processes in four different kraft pulp mill. (Keitaanniemi, 1979).

Metal Amount [kg/t]

Ca 16

Si 1.3

K 1.2

Mg 0.5

Mn 0.2

Fe 0.1

Al 0.1

In the Keitaanniemi (1979) study, amounts of metals entered to the processes varied quite a lot between different kraft pulp mills and were depending also on the amounts of raw mate-rial used. The behaviour of metals in the processes were depending on the amount of metal entering to the processes as well as chemical and physical features of metal and compounds metals formed. These features were for example the metal solubility, volatility and reactivity.

(Keitaanniemi, 1979).

Keitaanniemi’s study also showed metals to enter to effluents from bleaching processes. Ta-ble 7 lists the amounts (kg/ADt) of metals in the effluent of two Ta-bleaching processes using pine as a raw material:

Table 7. Loading [kg/ADt] to effluent from two bleaching processes of kraft pulp mills. The raw material used in the pulping processes was pine.

Metal Load to effluent [kg/ADt]

Ca 0.97

Si 0.22

K 0.2

Mg 0.34

Mn 0.06

Fe 0.03

Al 0.03

In addition to Table 7, BREF for pulp and paper industry lists typical metals and their amounts entering to effluents from kraft pulping processes where pulp is bleached or un-bleached. These metals listed in Table 8 are also included to this study.

Table 8. Typical loads [kg/ADt] of metals entering to effluents from kraft pulp mills. (Suhr et al. 2015).

Metal Load [kg/ADt] unbleached pulp Load [kg/ADt] bleached pulp

Pb 0.0003 0.0004

Ni 0.0004 0.0009

Cd 0.00003 0.0001

Cu 0.0005 0.001

Zn 0.005 0.015

Cr 0.0002 0.0007

In the combined wastewater treatment plant, wastewater from paper making and pulping contains heavy metals too. The total load from the combined wastewater treatment plant to the ocean is calculated as a sum of load from wastewater effluent and UPM drainage water discharge channel. The total load [kg/a] of the metals arsenic, mercury, cadmium, chromium, copper, lead, nickel and zinc in 2017 was following:

Figure 9. Metal load to the ocean from the Rauma mill area in 2017. Total load is calculated as a sum of a load from the combined wastewater treatment plant and UPM drain-age water discharge channel. Load from Metsä Fibre draindrain-age discharge channel is included already to the stream from the wastewater plant. Mercury Hg has not ex-ceeded its LOQ-value. (UPM wastewater measurements, 2017).